The invention relates to a humidity sensor for capacitive measurement of the humidity contained in a material, predominantly bulk products, also provided with a casing which exhibits a non-conductive closer plate whose outer side is turned towards the material or bulk products or over which material or bulk products travel, and with a stray field condenser arranged in a casing for generation of an electrical stray field whose field lines penetrate the closer plate and the material or bulk products, wherein at least one pole of the stray field condenser is attached as a conductive layer on the side of the closer plate facing away from the material or bulk products.
A humidity sensor of this type is known from patent document DE 36 12 282 and its function is to measure the humidity of a bulk product flowing over a baffle plate which has previously been homogenized as to its density in a reproducible and consistent manner by dropping onto the baffle plate. The correspondingly employed humidity sensor is housed in a plastic casing with a closer plate and mounted on the baffle plate such that the plane of the slip face of the baffle plate is flush with the closer plate. On the side of the closer plate turned away from the bulk product, there is a stray field condenser whose condenser poles are configured by two concentric wire loops. Upon connection of an electrical voltage to the two wire loops, a stray field is formed between the wire loops which penetrates the closer plate and the bulk product flowing over the closer plate. The effect of this is that the bulk product acts in relation to these stray fields as a dielectrical body whose dielectrical constants are largely dependent on the humidity of the bulk product. Since the geometrical condition of the stray field condenser remains constant, it can be assumed that modifications in humidity are approximately proportional to the changes in capacity of the stray field condenser.
These capacity changes are analyzed, in this type of humidity sensor, on the basis tat the stray field condenser is connected with a fixed capacity as a voltage divider, in which context the voltage divider is supplied with power from a high-frequency AC source. Accordingly, a humidity-dependent high frequency voltage is present in the stray field condenser and can be electronically reprocessed. Re-processing is performed by a central analyzer which is connected via a connecting line to the humidity sensor.
It has been found valuable to employ the above-described humidity sensor in particular in the production of transportable concrete, since the concrete quality is also decisively dependent on adherence to the water/cement level and this level can be reliably determined only if the humidity of the additives is taken into account. Particularly high requirements in terms of measurement accuracy and sensitivity of the employed humidity sensors are imposed upon metering of several additives, because the inaccuracies of individual humidity sensors will increment each other. It has been found that in these cases the accuracy and sensitivity of known humidity sensors is not adequate for the production of high-quality transportable concrete. A further problem in parallel operation of several humidity sensors also arises from the fact that the central analyzer is no longer capable of analyzing the data quantities arising. Furthermore, the replacement of an individual humidity sensor in a parallel-operation mixing system is very laborious, because for this purpose the whole mixing function of the system has to be interrupted in order to re-calibrate the individual sensor.